1. Field of the Invention
Generally, the present invention relates to printed antennas used in combination with devices for wireless data communication, and, more particularly, to a printed monopole antenna and devices, such as WLAN devices, mobile phones and the like, requiring compact and efficient antennas.
2. Description of the Related Art
Currently great efforts are being made to develop wireless data communication devices offering a high degree of reliability at low cost. A key issue in this respect is the degree of integration with which a corresponding transceiver device may be manufactured. While for many applications, such as direct broadcast satellite (DBS) receivers and WLAN devices, this is of great importance due to cost-effectiveness, in other applications, such as mobile phones, mobile radio receivers and the like, low power consumption is of primary concern.
Presently, two major architectures for receiver devices are competing on the market, i.e., the so-called direct conversion architecture and the so-called super-heterodyne architecture. Due to the higher degree of integration and the potential for reduction of power consumption, the direct conversion architecture seems to have become the preferred topography compared to the super-heterodyne architecture. However, the advantages achieved by improving the circuit technology may become effective, irrespective of the circuit architecture used, only to an extent as is determined by the characteristics of an antenna required in the high frequency module of the device, wherein the size, the radiation characteristic and the involved production cost of the antenna are also essential criteria that have a great influence on the economic success of the wireless data communication device.
In a typical wireless application, such as wireless data communication system using a local area network (LAN), usually the relative locations of communicating devices may change within a single communication session and/or from session to session. Hence, efficient methods and means have been developed to enhance reliability of the data transfer even for extremely varying environmental conditions, such as in the field of data communication with mobile phones. The overall performance of the wireless devices is, however, determined to a high degree by the properties of the antenna provided at the input/output side of the device. For instance, changing the orientation of a device may significantly affect the relative orientation of the polarization direction of the transmitter with respect to the receiver, which may result in a significant reduction of the field strength received in the receiver's antenna. For instance, changing the orientation of an initially horizontally radiating dipole antenna into the vertical orientation may lead to a reduction of the voltage generated by a horizontally oriented receiver antenna up to approximately 20 dB. Consequently, for non-stationary applications in the wireless data communication system, a substantially isotropic radiation characteristic, independent of the polarization direction, is desirable. On the other hand, with respect to portability and usability of the wireless devices, it is generally desirable that antennas for wireless data communication systems occupy as little volume within the device as possible and to substantially avoid design modifications in the form of, for example, protruding portions and the like. Therefore, increasingly, antennas are provided, which are printed onto a dielectric substrate and connected to the drive/receive circuitry, wherein, in recent developments, the antenna is printed on a portion of the same substrate that also bears the system circuit. Although a moderately compact antenna design is achieved by conventional printed antennas, it turns out to be difficult to provide a highly isotropic characteristic of a dipole antenna when printed on a circuit board.
Thus, great efforts are made to provide efficient and small printed antenna designs with a desired isotropic radiation characteristic. Frequently, a monopole design is used for small volume devices, since the length of the resonant path of a monopole antenna requires only to be equal to a fourth of the wavelength of interest compared to half of the wavelength as is typically used for dipole antennas. The ground plane necessary for producing the mirror currents in a monopole architecture may often be provided without consuming undue substrate area, thereby rendering the monopole antenna an attractive approach for small-sized devices. In IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, Vol. 51, No. 9, September 2003, a double T-shaped monopole antenna is described, wherein the length of the resonant paths are selected to enable a dual band operation at 2.4 GHz and 5.2 GHz, respectively. However, the radiation characteristic of the double T antenna with respect to applications requiring a high degree of isotropy is not discussed.
Therefore, a need exists for a printed monopole antenna exhibiting high performance with respect to a desired spatially isotropic radiation characteristic while allowing a low cost and low size design.